1. Field of the invention
The present invention relates to video receivers in general and in particular to circuits for automatically searching broadcasting stations within a certain frequency band. The invention is particularly useful for video receivers though it may be also utilized in similar receivers.
2. Description of the prior art
An automatic search procedure for a broadcasting station consists in having the local oscillator or the frequency synthesizer of the tuner of a receiver perform a frequency excursion, from low frequencies to high frequencies or vice versa within a certain frequency band wherein broadcasting stations are to be found. The frequency excursion (continuous or more commonly carried out by a succession of equal increments of frequency) must be automatically arrested when a carrier is detected, in order for the operator to verify whether such a carrier belong or not to the desired broadcasting station and eventually to order the prosecution of the automatic search until another carrier is detected, and so on. In modern receivers this automatic search procedure is usually controlled by a microprocessor.
In video receivers it is a common practice to cause the arrest of the incremental frequency excursion of the local oscillator or synthesizer during an automatic search procedure for broadcasting stations by using synchronism and/or the so-called "fly-back" signal generated by the horizontal scanning yoke for obtaining a stop signal of the search scan; the detection, in the demodulated signal at the output of the intermediate frequency (IF) stages of the video receiver, of synchronism signals alerting that the tuner of the receiver is in tune with a video carrier.
On the other hand, in modern video receivers, at the output of the intermediate frequency stages, i.e. at the output of the intermediate frequency demodulator, there is usually a "SCART" socket for the connection of an eventual video recorder. The known system for arresting the search scan for broadcasting stations has the disadvantage that, for example during the play through the television set of a recorded video program, the tuning section and the intermediate frequency conversion and demodulation section thereof cannot be exploited independently for searching broadcasting stations and for receiving programs which could be simultaneously registered by means of a second video recorder connected thereto.
Notably, almost all tuners are provided with an automatic frequency control circuit (AFC) which ensures automatically a perfect and stable tuning once the tuner is brought within a certain tuning interval of frequencies, whose limits are: f.sub.o -.DELTA.f and f.sub.o +.DELTA.f, where f.sub.o is the central carrier frequency and Af is a pre-established frequency interval. The AFC system substantially utilizes a circuit which compares the frequency of the signal applied to an input thereof with a standard video intermediate frequency (i.e. 38.9 MHz for television sets operating in accordance with the B/G standard); when the frequency of the input signal is equal to the IF, the voltage generated at an output terminal of the AFC circuit may be equal to a certain fractionary value (V.sub.s /x) of the supply voltage Vs, e.g. V.sub.s /2 (commonly 6V); when the frequency of the input signal is lower than the IF, the output voltage generated by the circuit becomes lower than such a pre-established fractionary value of the supply voltage or higher when the frequency of the input signal is higher than the IF.
The voltage-frequency characteristic (AFC signal) of such a AFC circuit has the well-known "S" shape, centered on the frequency f.sub.o of a video carrier and is depicted in FIG. 1.
This characteristic, i.e. the AFC signal level curve in function of frequency, may be run through in either direction during a frequency scan by the local oscillator or synthesizer for the automatic search of broadcasting station, i.e. from low frequencies to high frequencies or vice versa.